The present invention relates to an antenna feeder system for circularly polarized signals, the system including an exciter whose aperture cross section is symmetrical with respect to at least one major axis and a device for coupling a plurality of wave modes, such as higher order modes as divergence-indicating signals for positioning the antenna in that excitation is effected proportionally to the divergence of the major axis of the antenna from the direction of a received circularly polarized beacon signal.
One property sought for communications satellites is that they cover a precisely defined area on the earth and affect adjacent areas as little as possible, particularly where the supply of television programs to only one of two adjacent countries is concerned.
In order to prevent a radiation field emitted by a satellite antenna from drifting to adjacent areas, the alignment of the transmitting antenna must be stabilized. The paper by applicant entitled "Analyse und Synthese von elektromagnetischen Wellenfeldern in Reflektorantennen mit Hilfe von Mehrtyp-Wellenleitern" [Analysis and Synthesis of Electromagnetic Wave Fields in Reflector Antennas with the Aid of Multiple Mode Waveguides] Dissertation D82, RWTH-Aachen (1978), pages 46 et seq., discloses, for example, such a transmitting antenna which operates as a monopulse sensor.
This transmitting antenna simultaneously serves as a receiving antenna for a beacon signal which is transmitted by a beacon station disposed in the center of the prescribed broadcast area. In dependence on the deviation of the major axis, i.e., the axis of the radiation pattern, of the exciter of the satellite transmitting antenna from the received beacon signal, higher order wave modes are excited in the transmitting antenna. These modes are coupled in by means of a mode coupler disposed directly behind the exciter and are used as deviation signals. The beacon signal employed here is a linearly polarized signal.
However, the antenna feeder system to be discussed below is a system including a device for coupling in higher wave modes as deviation signals for circularly polarized signals wherein the exciter may also have a shape which is symmetrical only with one major axis of the aperture surface so as to produce, for example, an elliptical illumination area at the earth's surface. A further prerequisite to be considered in the present system is that the frequency of the received signal which is composed of the beacon signal and possibly an additionally transmitted communications signal, is much greater than the frequency of the transmitted signal (f.sub.rec =17.3 to 18.1 GHz, f.sub.tr =11.7 to 12.5 GHz). Because of the requirement that f.sub.rec &gt;&gt;f.sub.tr, it is possible to couple the higher order modes into the exciter only with difficulty since the exciter throat cannot be made small enough to force the higher modes to be totally reflected, which is a prerequisite for selectively coupling in the higher order modes. Otherwise, a very complicated and cumbersome coupling device is required. Such a coupling device is disclosed, for example in German Auslegeschrift No. 2,608,092 and corresponding U.S. Pat. No. 4,048,592.